Commodore 64: Cyberload F3 loader

@DrWatson

Published in 

C64 tape formats

 · 3 years ago

The loader is contained in the first F2 file.

Files of this format will contain the actual game binaries, they are formatted as a sequence of sub-blocks, each one contains 256 bytes or less.

Surprisingly they are NOT enciphered in any way.

---------------

Threshold : Varies between tapes.
Endianess : MSbF

Pilot byte : Varies between tapes (commonly $0F) (*8)
Sync byte : Varies between tapes (commonly $CC).

Then...

2 bytes : Load address (LSBF).
1 byte : Data size (n), if $00 then n = 256.
1 byte : Checkbyte (see notes below).
1 byte : internal use.
1 byte : internal use.
1 byte : internal use.
1 byte : internal use.
n bytes : Data. (n= 256 or less).

Notes:

Typically these sub-block files make up the majority of a tapes contents, there are usually a large number of them chained one after the other, sometimes they are separated by pauses, sometimes not.

The checkbyte in the header is the result of 0 XOR'ed with ALL bytes in the sub-block except itself